Field of the Invention
The present invention pertains to compliant foil gas bearings, and more particularly, an air bearing arrangement for an electrically driven compressor.
Background of the Invention
Conventionally, air bearings have a cylindrical bush, at least one undulatingly bent metal sheet (bump foil) anchored at the leading edge in a slot in the bush, and at least one substantially smooth metal sheet (top foil) bent into a cylindrical shape slightly larger than the outer diameter of a rotary shaft and anchored at the trailing edge in a slot in the bush. During operation, an air lubrication layer forms between the rotor and the top foil, which air film permits sliding between the rotor an the top foil and allowing the bearing to effectively support a high bearing load. Air bearings are especially attractive in turbochargers because they eliminate the lubricating oil system, simplify the seal gas system, simplify the control system, offer lower frictional power loss, and provide greater flexibility of machine installation.
Such devices are shown in U.S. Pat. Nos. 3,893,733, 4,133,585, 4,167,295, 4,262,975, and 4,300,806. According to U.S. Pat. No. 4,549,821 (see Prior Art FIG. 1), when a resting rotary shaft 1 is located within an axial bore defined by the inner surface of the top foil 5 in the foil bearing, the center axis of the rotary shaft is slightly offset from the axis of the top foil. A wedge shaped gap is formed in the narrow space between the inner surface of the top foil and the outer surface of the rotary shaft. The motion of the rotating shaft applies viscous drag forces to the fluid (air) in the converging channel, resulting in a fluid pressure increase throughout most of the channel. The nearer to the narrow end of the wedge-shaped space, the higher the pressure of the air. The rotary shaft is forced by this higher pressure of air to return to the center axis of the top foil. Therefore, when the rotary shaft is rotated at a high speed, such a force acts to return the rotary shaft to the center axis of the top foil at all times. Air foils provide a cushioning and damping effect, functioning to accommodate eccentricity, run-out and other non-uniformities in the relative movements of the relatively moveable elements.
The air bearing may comprise one, two, or more top foils or “pads”, e.g., each covering 120° of the circumference respectively in the case of three pads, each pad forming a wedge-shaped fluid-dynamic wedge channel, thus centering the shaft from three sides.
A small circumferential gap 8 forms between the successive top foils 5. Ideally, air leaving an “upstream” top foil 5 should flow across the circumferential gap 8 into the radial gap 7 between rotary shaft 1 and top foil 5 of a subsequent or “downstream” pad to produce the air pressure to center the shaft. However, in the case of a conventional top foil 5 with straight leading edge 5d (see prior art FIG. 1, derived from U.S. Pat. No. 4,549,821 discussed above), there is the possibility that air leaving an upstream preceding pad flows into the circumferential gap 8 between the pads and thence into the radial gap 7 between top foil 5 and bump foil 4, generating pressure between the bump foil 4 and the top foil 5. As a result, the top foil 5 will bear strongly against the rotor 1, triggering a suction effect in the gap between rotor 1 and top foil, which will further enhance contact, causing severe wear of the top foil 5 and loss of load bearing capacity and ultimately bearing failure.